In a wireless charging system, a wireless power adapter may use a coil to wirelessly transmit power to a wireless power receiving device. The wireless power receiving device may have a coil and a rectifier. The coil receives wirelessly transmitted power signals from the wireless power adapter. The rectifier converts the received signals into direct-current (DC) power.
It may sometimes be desirable to transmit data from a wireless power receiving device to a wireless power adapter. So-called in-band communications schemes have been developed that allow wireless power receiving devices to communicate with wireless power devices. In a typical in-band communications scheme, a switching circuit that is coupled to the coil in the receiving device is used to modulate the load across the coil. The wireless power adapter can detect the modulated signal using a voltage sensing circuit coupled to a coil in the wireless power adapter.
In-band communications schemes that are based on this type of load modulation may not always be reliable. For example, if electromagnetic coupling between the wireless power adapter and the wireless power receiving device is poor, the voltage sensing circuit in the wireless power adapter may not be able to detect the modulated signal from the wireless power receiving device.